What Is BMI?
Generally, BMI is used to determine if you have a healthy body weight for your height.(2) BMI is intended to determine tissue mass. It helps you understand if you are overweight, underweight, or at a healthy weight. Healthy BMI ranges vary depending on your age. BMI is a generic measurement intended to be used with other factors to offer information about your health. It is a useful indicator to determine if you need to lose or gain weight. Since being over- or underweight can have significant health effects, knowing your BMI is important.
What Is a Healthy BMI?
For adults over the age of 20, BMI ranges are as follows:(1)
- Less than 16: Severely underweight
- 16 -17: Moderately underweight
- 17-18.5: Mildly underweight
- 18.5 - 25: Normal
- 25 - 30: Overweight
- 30 - 35: Obese class I
- 35 - 40: Obese class II
- Greater than 40: Obese class III
Healthy BMI ranges for children depend on where your child falls on the growth chart. Normal BMI ranges for children are found below:
- Below the 5th percentile: Underweight
- Equal to or greater than the 5th percentile and less than the 85th percentile: Normal weight
- At or above the 85th percentile but below the 95th percentile: Overweight
- At or above the 95th percentile: Obese
The Risks of Being Overweight
If your BMI indicates that you are overweight, you may be at risk of developing certain health conditions. Risks associated with being overweight include:
- Hypertension or high blood pressure
- Elevated triglycerides, also known as "bad" cholesterol
- Developing type 2 diabetes
- Heart disease
- Gallbladder disease
- Sleep apnea and breathing issues
- Some types of cancer
- Mental illness
- Increased risk of mortality
Keeping your BMI at or below 25 kg/m2 can help you avoid the above risks. Losing weight is challenging, but it is entirely worth the effort required to increase the quality of life and improve overall health. If your BMI is higher than you’d like to see, you should work with your doctor to determine if you have any other health issues so you can find a realistic and feasible weight loss plan.
The Risks of Being Underweight
Maintaining a healthy BMI should not be interpreted as keeping your BMI as low as possible. Just as there are risks associated with your BMI being too high, there are risks associated with it being too low. Some risks of being underweight include:
- Vitamin and mineral deficiencies
- Bone weakness and breaks caused by osteoporosis
- Low immune function
- Developmental issues in children and teens
- Hormonal imbalances and irregular menstrual cycles in women
- Surgical complications
- Increased risk of mortality
Being underweight can sometimes be a sign of an underlying health condition. It is important to speak with your doctor if you have an abnormally low BMI so that they can check for other health issues and determine a feasible way for you to start gaining weight.
Even though BMI is a useful indicator of health, it is a very general measure and should be used alongside other assessments to gain a fuller picture of a person’s health. BMI is only an estimate, and it does not take those other important factors into account—such as the distribution of muscle, bone mass, and fat.
For adults, BMI lacks accuracy when used on its own because it is a measure of weight, not fat. Your age, muscle mass, body fat, activity level, and sex all influence your BMI and should be considered when assessing your health. For example, an inactive older person may have a “healthy” BMI, yet they are not actually healthy since they lack muscle.
Conversely, many athletes would be considered overweight based on their BMI because of their greater muscle mass. You should never look at BMI alone to determine if you are healthy or to guide healthcare decisions. Health is a complex concept, and BMI is only one of many factors to consider.
To calculate your BMI, you can use an online calculator where you simply plug in your height and weight to find out your BMI.(1) You can also use a BMI formula to do it manually.
The formula for calculating BMI uses the metric system:
- BMI = weight in kilograms/height 2 in meters
For example, if you weigh 75 kilograms and are 1.9 meters tall, this is how you would calculate your BMI:
- 75 (1.9) 2 = 75 3.61 = 20.78
To use U.S. measurements (pounds and inches) to calculate your BMI, apply the following formula:
- BMI = 703 x weight in pounds/height 2 in inches
For example, if you weigh 125 pounds and are 70 inches tall, this is how you would calculate your BMI:
- 703 x (125 (70)2) = 703 x (125 4,900) = 703 x 0.0255 = 17.93
How do I calculate BMI?
A BMI calculator is an easy way to determine your BMI. All you have to do is figure out your weight and height. Make sure you’re using the correct units of measure. Some BMI calculators use the metric system (centimeters and kilograms), while others use U.S. measurements (inches and pounds). You can calculate your BMI manually with the BMI formula (above), but a calculator will be the easiest way.
What is the best BMI for my age?
If you are over 20, a normal BMI is between 18.5 and 25. If you fall outside of range, you may be considered overweight or underweight for your age. Your BMI is only one measure of health, so it is important to work with your doctor in determining if you need to lose or gain weight.
What is a healthy BMI for a woman?
A normal BMI for adult women over the age of 20 is 18.5 to 25. Women tend to have more body fat than men, so a healthy BMI can be associated with unhealthy fat levels when compared with a man.
What BMI range is normal?
A normal BMI range for adults is 18.5 to 25, but you may still be considered healthy if your BMI is slightly outside of this range, depending on your percentages of muscle mass and body fat.